Operating an engine with more than a single fuel allows an engine to operate in a way that may improve engine operation as compared to when only a single fuel type is available. For example, an engine operating with reformed fuel may be able to tolerate a higher level of cylinder dilution than an engine operating solely with gasoline. On the other hand, it may be desirable to operate the engine solely with gasoline when reformate is unavailable or in low supply. Thus, it may be desirable to adjust operation of an engine depending on an amount of available fuel. U.S. Patent Application 2008/0221778 describes a system wherein engine speed and load are set differently when a quantity of fuel stored in a second fuel tank is less than a predetermined value.
While it may make sense to adjust engine speed and load in response to an amount of fuel in a fuel tank, simply limiting engine speed and load may not leverage the remaining fuel in a way that improves engine operation with the remaining fuel. Further, although it may be desirable to limit engine speed and load, such limiting may provide little benefit if the engine is operating at high dilution levels.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for improving engine operation. One embodiment of the present description includes a method for operating an engine, comprising: reforming a first fuel into a gaseous fuel; operating said engine by injecting said gaseous fuel and a second fuel to a cylinder of said engine in response to an available amount of said gaseous fuel, engine speed, and engine load; and adjusting an engine actuator to vary cylinder charge dilution in response to said available amount of said gaseous fuel.
By adjusting cylinder charge dilution in response to an available amount of gaseous fuel it may be possible to extend engine operation at higher dilution levels. For example, it may be desirable to reduce the amount of gaseous fuel injected to engine cylinders and to reduce cylinder charge dilution from a first amount to a second amount at particular engine operating conditions to conserve gaseous fuel when the amount of gaseous stored in a storage tank is less than a predetermined amount. Reducing the amount of injected gaseous fuel may allow the engine to operate at higher dilution levels for a longer period of time. Further, cylinder charge dilution may be adjusted in response to the type of fuel stored in the gaseous fuel storage tank to take advantage of unique fuel attributes. For example, if the gaseous fuel is comprised of vaporized alcohol, the engine may be operated at a first dilution level. On the other hand, if the gaseous fuel is comprised of reformate, the engine may be operated at a second dilution level, the second dilution level greater than the first dilution level.
The present description may provide several advantages. Specifically, the approach may reduce engine emissions and improve fuel economy by better utilizing gaseous fuel. Further, the method may allow a smaller fuel reformer to provide reformate to the engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.